Biodiesel (biofuel) is a liquid obtained from natural lipids such as vegetable oils or animal fats, with or without prior use, through industrial processes of esterification and transesterification, and which is applied in the preparation of total or partial substitutes of the diesel or gasoil from petroleum. Biodiesel can be mixed with gas oil from the refining of petroleum in different quantities. Abbreviated notations are used according to the percentage by volume of biodiesel in the mixture: B100 in case of using only biodlesel, or other notations such as B5, B15, B30 or B50, where the numerals indicate the percentage by volume of biodiesel in the mixture.
Currently, the production of biodiesel is increasing and therefore the search for catalysts for its production has deserved a great development. The reaction to produce biodiesel consists of the transesterification of a molecule of triglyceride ester and three molecules of alcohol, which is typically methanol, to generate three molecules of fatty acid methyl ester FAME, which is biodiesel and a molecule of glycerine, accordingly to the following reaction scheme:

The process of transesterification of the triglyceride esters with methanol can be described in three steps: the triglyceride generates a fatty acid methyl ester molecule, FAME, and a diglyceride. The diglyceride in turn generates another molecule of FAME and a monoglyceride, which in turn generates the third molecule of FAME and glycerine. This sequence of reactions is carried out in the presence of basic catalysts which act on the alcohol to increase its attack reactivity or by acid catalysts acting on the oxygen of the ester in the triglyceride or the diglyceride or monoglyceride to activate the attack of the alcohol.
In the article Advances in Heterogeneous Catalysis for Biodiesel Synthesis, Top Catal. 53, 721-736, 2010, Yan et al., describe both the limitations of homogeneous or first generation catalysts, which act in the same reaction phase as the advantages of heterogeneous or second generation catalysts.
The limitations of the homogenous or first generation catalysts are:    a) their use is normally limited to batch processing, and are essentially highly corrosive acids or bases;    b) the stages of the homogeneous biodiesel production process are time consuming and highly expensive to process because the process comprises the steps of: oil pretreatment, catalytic transesterification, fatty acid methyl ester (FAME)/glycerin separation, neutralization of the residues of the homogeneous catalyst, distillation of methanol, washing with water of the FAME phase, and vacuum drying of the desired products;    c) it is impossible to reuse the homogeneous catalyst in reaction cycles as it is lost in the waste streams;    d) separation of the products requires a post-treatment with large volumes of water to neutralize the catalyst residues which generates waste water that must be treated before its release into the environment;    e) the homogeneous catalyst is sensitive to the free fatty acids (FFA) and water present in vegetable oils.
The advantages of heterogeneous or second generation catalysts are as follows:    a) The catalyst is not lost, it can be recovered from the reaction medium and subjected to several reaction cycles;    b) can be used in packed-bed reactors for continuous flow processes;    c) The post-treatment of product is not required or reduced, significantly reducing the ecological impact by avoiding the liquid wastes generated by the purification of the products; and    d) The microcrystalline structure of the catalyst surface is stable, which extends its useful life.
Acid type catalysts may be defined as oxygen carbonyl activators of the ester to increase its reactivity against the attack of the alcohol, typically methanol. These acid catalysts can be classified in those with Brönsted acid sites because they have interactions of carbonyl oxygen with catalytic proton (H+) sites and those with Lewis acid sites because of interactions of carbonyl oxygen with cationic (M+) sites in the catalyst
In FIG. 1 the above is presented, emphasizing experimental evidence showing the type of acidity on the surface of the catalyst by an infrared spectrum which describes the adsorption of a Lewis base, such as pyridine, on an acid surface. The interaction of the acid sites on the surface with the pyridine molecule generates different bands, being the characteristics for Brönsted acid sites those that appear at 1,545 cm−1 and the characteristics for Lewis acid sites appear at 1,445 cm−1 and the intermediate of 1,490 cm−1 for both interactions.                section (a) of FIG. 1 shows how the acid catalyst with Brönsted (H′) sites acts on the triglyceride molecule activating it for the transesterification reaction;        in section (b) of FIG. 1 it is shown how the acid catalyst with Lewis (M′) sites acts on the triglyceride molecule activating it for the transesterification reaction; and        section (c) of FIG. 1 shows a catalyst having both types of sites.        
These interactions are also present in diglycerides and monoglycerides activating them for transesterification reactions with an alcohol, such as methanol. Thus, we can say that the catalysts of acidic nature to activate the triglyceride, diglyceride or monoglyceride molecules act at the promoter level of the C═O ester.
Catalysts with Brönsted acidity, such as heteropolyacids (HPAs), have shown high catalytic activity, yield and conversion, in combination with monovalent cations, such as Cs+. The HPAs based on Nb and W supported on W—Nb, tungstated zirconia, tantalum pentoxide and silver have shown more resistance to catalyst leaching, as described in the following bibliographic citations.
Katada N. et al., in Applied Catalysis A General, (2009), 363 (164: 168); studied solid acid catalysts derived from HPAs with W and Nb. They found that under a calcination temperature of 773K, W and the HPAs based on Nb and supported on WO3-Niobia (WO3—Nb2O5) are transformed into NPNbW/W—Nb formulations Insoluble in the reaction mixture and with high catalytic activity for the transesterification of triolein and ethanol to ethyloleate. The reaction rate is increased when methanol is used instead of ethanol. Due to the potential and catalytic stability during at least 4 days of reaction of these catalysts, the authors recommend carrying out the reaction in fixed-bed and continuous flow.
Shi et al., in Chemical Engineering & Technology (2012), 35 (2), 347-352, describe that HPAs were used as triglyceride transesterification catalysts, arguing for both Brönsted and Lewis acidity properties. HPAs that are strong Brönsted acids, depending on their composition and the reaction medium, possess good thermal stability, high acidity and high oxidizing capacity and are water tolerant. Among HPAs, 12-tungstophosphoric acid (H3PW12O40) is chosen because of its high activity, since it shows a Keggin structure which is composed of a coordinated tetrahedral heteroatom of oxygen (PO4) surrounded by 12 additions of atoms sharing oxygen atoms coordinated octahedral (WO6) according to Oliveira C. F. et al. (Esterification of oleic acid with ethanol by 12-tungstophosphoric acid supported on zirconia. Appl Catal A-Gen 372: 153-161 (2010)).
Heterogeneous catalysts of acidic nature such as those based on sulfated zirconia have been reported, emphasizing that the carbonyl oxygen activating acid sites are of the Brönsted nature according to Rattanaphra et al., “Simultaneous Conversion of Triglyceride/Free Fatty Acid Mixtures into Biodiesel Using Sulfated Zirconia”, Top Catal. 53: 773-782, 2010.
On the other hand, the esterification of palmitic acid with HPA catalyst was also carried out by Caetano et al. (Esterification of free fatty acids with methanol using heteropolyacids immobilized on silica. Catal Commun 9:1996-1999 (2008)), using heterogeneous catalysts: Tungstophosphoric acid (PW), molybdophosphoric acid (PMo), and tungstosilicic acid (SiW) immobilized in silica by the sol-gel technique, of them the PW proved to be the best catalyst for which it was studied with different concentrations of load in silica, obtaining 100% conversion of the palmitic acid with a concentration of 0.042 g PW/g silica.
Zeolites, on the other hand, because of their uniform pore structure, have clear advantages of having a system with interconnected pores, so that the entire surface of the solid is available for promoting the transesterification reaction. The surface must be hydrophobic to promote the preferential adsorption of hydrophobic fats on the surface of the catalyst and to avoid deactivation of the catalytic sites by the strong adsorption of polar compounds such as glycerin or water. Z. Helwani, M. R. et al., (Applied Catalysis A: General 363 (2009) 1-10) Review: Solid heterogeneous catalysts for transesterification of triglycerides with methanol: A review).
Alternatively, in the patent document by Tian et al., CN 103801282, “Solid base catalyst, and preparation method and application thereof”, the use of an aluminum-Zn spinel catalyst (ZnAlxO1+1.5x where x=1.5-2.5) doped with La is described. The basic solid catalyst is used in the transesterification reaction of fatty acid esters with an alcohol to produce biodiesel; is of high and stable activity during its use, and the active components are not lost.
In the patent document CN 103,752,297 “Zirconium-oxide catalyst for producing biodlesel, as well as preparation method and application of zirconium-oxide catalyst. 2014” a zirconium oxide catalyst is claimed to produce biodiesel in a tubular reactor at a reaction temperature of 250-300° C., reaction pressure of 7 to 14 MPa and volume ratio of alcohol-oil of 0.5:1 to 7:1. The catalyst is characterized by containing zirconium oxide of 80-95 weight %, aluminum oxide of 2-18 weight %, 1 to 17% titanium dioxide, 5 to 25% sodium bicarbonate and 10 to 50% sodium chloride; and in the patent document CN 103,706,384 “Preparation method of bio-diesel catalyst. 2014”, there is provided a method of preparing a catalyst for the production of biodiesel in a continuous flow process in which the composition is PO43−/ZrO2 doped with rare earth metals such as La, Ce, Pr, Nd, etc.
The Institut Français du Petrole has developed an industrial level technology called Esterfip-H™, which refers to a continuous process of transesterification where the reaction is promoted by a heterogeneous catalyst, which is a zinc aluminate (ZnAl2O4) spinel, which promotes the transesterification reaction, without loss of catalyst. The reaction is carried out at an operating temperature of 180-220° C. and pressure of 40-60 bar (40.79-61.18 kg/cm2). The yields obtained are greater than 98%, with an excess of methanol. However, the raw material must have a free fatty acid content lower than 0.25% and a water content lower than 1,000 ppm. (Juan A. Melero et al., Critical Review, Heterogeneous acid catalysts for biodiesel production: current status and future challenges, Green Chem., 2009, 11, 1285-1308).
In U.S. Pat. No. 5,908,946, “Stem R. et al., Institut Français du Petrole, Process for the production of esters from vegetable oils or animal oils alcohols” (1999), for the production of esters of linear monocarboxylic acids with vegetable oils of 6 to 26 carbon atoms or oils of animal origin are reacted with monoalcohols having a low molecular weight, for example of 1 to 5 carbon atoms, In the presence of a catalyst selected among of zinc oxide, mixtures of zinc oxide and aluminum oxide, and the zinc aluminates corresponding to the formula: ZnAl2O4, xZnO, and Al2O3 (with x and y being 0-2 each) and with a spinel-type structure, allowing the direct production in one or more stages of an ester that can be used as fuel and pure glycerin. To process vegetable oil, severe operating conditions are considered: temperatures of 170-250° C., pressures lower than 100 bar (101.97 kg/cm2), with excess stoichiometric alcohol, achieving conversions of 80-85%. In the case of acid oil feeds, operating conditions of 180-220° C. are used, with pressures lower than 1 bar (1.02 kg/cm2).
Considering metal phosphates as metal catalysts for transesterification of biodiesel, Xie et al., in Bioresource Technology (2012), 119, 60-65, describe acid catalysts for transesterification of triglyceride esters based on 30 wt % WO3 supported in AlPO4, which were tested in batch reaction systems at 180° C. for 5 h and a methanol/oil ratio of 30:1 at a dose of 5 weight % catalyst.
Other catalysis systems for transesterification consist of calcium phosphates from pyrolysis of animal bone, which generates hydroxyapatite at 800° C., as described by Obadiah et al., in Bioresource Technology (2012), 116, 512-516.
Also, in the patent document CN 103,484,258 “Method for preparing biodiesel by using nano hydroxyapatite to catalyze triglyceride 2014”, a method is described for preparing biodiesel in the presence of a nanohydroxyapatite catalyst in concentration of 0.5 to 3 weight % and operating from 800 to 300° C. for 2 to 10 h.
Yin et al. Describe in Fuel (2012), 93, 284-287 the catalytic activity of K3PO4 at conditions of 220° C., a methanol-oil ratio of 24:1 and 1% of the catalyst resulting in a conversion of 95.6%.
Sodium phosphate has also been used as a transesterification catalyst for triglyceride esters in biodiesel production according to De Fillipis et al., In Energy & Fuels (2005), 19 (6), 2225-2228.